We are studying the structure and genetic behavior of human and mouse ribosomal genes. Using recombinant DNA technology, we have purified DNA fragments containing rDNA sequences from both species. We are continuing these experiments in order to obtain DNA sequences representing the whole ribosomal gene repeating unit. We are studying the genetic behavior of these sequences as they are propagated in E. coli since we have already shown that the genetic behavior of portions of mouse ribosomal gene non-transcribed spacer when grown in E. coli can mimic those events that have gone on during mouse rDNA evolution. Our studies also include the chromosomal localization of the polymorphic forms of human rDNA using mouse-human hybrid cells and an analysis of the evolution of rDNA sequences in primates.